This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:    3GPP third generation partnership project    C-plane control plane    CK_cs CS cipher key    CK_ps PS cipher key    CS circuit switched    EDGE enhanced data rates for GSM evolution    E-UTRAN evolved universal terrestrial radio access network    GERAN GSM/EDGE radio access network    GGSN gateway GPRS support node    GPRS general packet radio service    GSM global system for mobile communications    HSPA high speed packet access    IK_cs CS integrity key    IK_ps PS integrity key    IMS internet protocol multimedia subsystem    MSC mobile switching center    NAS non-access stratum    NW network    PS packet switched    RAB radio access bearer    RAT radio access technology    RNC radio network controller    SGSN serving gateway support node    SK security key    SN sequence number    SRB signaling radio bearer    SRNS serving radio network subsystem    SRVCC single radio voice call continuity    UE user equipment    U-plane user plane    (U)SIM (UMTS) subscriber identity module    UTRAN universal terrestrial radio access network    UMTS universal mobile telecommunications system    VLR visited location register    VoIP voice over internet protocol
SRVCC operation is described by 3GPP TS 23.216 V9.3.0 (2010-03) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Single Radio Voice Call Continuity (SRVCC); Stage 2 (Release 9).
As is defined in 3GPP TS 23.216, single radio voice call continuity refers to voice call continuity between IMS over PS access and CS access for calls that are anchored in the IMS. The UE is assumed capable of transmitting/receiving on only one of these access networks at a given time. A 3GPP SRVCC UE is defined as a UE that is enhanced for IMS service continuity with the additional UE capabilities described in 3GPP TS 23.216 for SRVCC between E-UTRAN and 3GPP UTRAN and/or between E-UTRAN and 3GPP GERAN and/or between UTRAN (HSPA) and 3GPP UTRAN and 3GPP GERAN.
SRVCC allows a voice call to be handed over from a PS connection to a CS connection. As can be appreciated, this is essentially a handover from a VoIP call over a data bearer to a traditional voice call over a circuit switched bearer. The SRVCC feature allows the rollout of VoIP by providing interworking between old and new systems.
Reference can also be made to 3GPP TS 33.102 V9.2.0 (2010-03) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; 3G Security; Security architecture (Release 9), and to 3GPP TS 33.401 V9.3.1 (2010-04) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; 3GPP System Architecture Evolution (SAE); Security architecture (Release 9). For example, 3GPP TS 33.401, Section 14, “SRVCC between E-UTRAN and Circuit Switched UTRAN/GERAN” references 3GPP TS 23.216.
However, 3GPP TS 23.216 does not deal with at least some security issues associated with the handover between NAS domains. In particular, methods for key handling are absent.